Such a vortex flow meter is described in U.S. Pat. No. 4,716,770. The conduit of the measuring tube contains a bluff body which is designed to generate Kaman vortices and has a blind hole which is in communication with the conduit via passages formed in the bluff body. Inserted in the blind hole is a vortex sensor with a capacitive transducer which is responsive to the vortex-induced pressure fluctuations and comprises a first and a second oscillating body, the first oscillating body being a sensor sleeve which is deflectable by the pressure fluctuations, has a cavity sealed from the fluid, and supports a capacitor electrode within the cavity, and the second oscillating body being movably disposed within the cavity so as to be isolated from the pressure fluctuations and supporting at least one additional capacitor electrode. Connected to the capacitor/ capacitors is a specific measuring circuit working on the switched-capacitor principle.
Although thousands of the prior art vortex flow meters have proved good in practice, the capacitive sensor, on the one hand, is not operable as a floating device because of the unilateral grounding of its capacitor electrode/electrodes and, on the other hand, does not allow a minimum power requirement of the measuring circuit. Particularly the charge reversals conditioned by the switched-capacitor principle and the power necessary therefor are disturbing in specific applications, e.g., if the requirements of the German standard DIN 19 230 (4-mA-to-20-mA technology) are to be met.
It has also turned out in practice that the manufacture of the prior art capacitive sensor is quite cost-intensive, since highly accurate mechanical processing steps, partly under clean-room conditions, are necessary. Also, the sensor is not always usable in the high-temperature range.
It is, therefore, an object of the invention to provide another sensor principle which is largely compatible with the prior art construction.